1. Field of the Invention
This invention relates to a stacked photovoltaic device, such as a solar cell and a photosensor, formed by superposing at least three p-i-n junction constituent devices, each having a p-type layer, an i-type layer and an n-type layer which are formed of silicon non-single crystal semiconductors.
2. Related Background Art
Photovoltaic devices which are photoelectric conversion devices that convert sunlight into electric energy are currently in wide use as public-purpose power sources for low-power supply, such as in electronic calculators and wrist watches. Such devices attract attention as practical future substitutes for petroleum fuel such as oil and coal.
Photovoltaic devices utilize photovoltaic force attributable to a p-n junction or the like of semiconductor devices. Semiconductors such as silicon absorb sunlight to produce photocarriers of electrons and holes. The photocarriers are caused to drift by the aid of an internal electric field at the p-n junction or the like and taken outside.
Such photovoltaic devices are produced chiefly by using semiconductor fabrication processes. Stated specifically, a silicon single crystal whose valence electrons have been controlled to be p-type or n-type is produced by a crystal growth process as exemplified by the CZ (Czokralski) process. The single crystal thus produced is sliced to prepare a silicon wafer about 300 μm thick. A layer having different conductivity type is further superposed by, e.g., diffusing a valence electron control agent so as to have a conductivity type opposite to that of the wafer to provide a p-n junction.
Now, at present, from the viewpoint of reliability and photoelectric conversion efficiency, single-crystal silicon is employed in most photovoltaic devices having been put into practical use. However, since the production of such single-crystal silicon photovoltaic devices makes use of the semiconductor production process as stated above, it entails a high production cost.
The single-crystal silicon photovoltaic devices have additional disadvantages in that the single-crystal silicon has a small optical absorption coefficient because of its indirect transition and hence must be formed in a thickness of at least 50 μm in order to absorb more sunlight and in that it has a band gap of about 1.1 eV, which is narrower than 1.5 eV suited for photovoltaic devices and hence cannot utilize short wavelength components effectively.
Even if polycrystalline silicon is used to achieve a cost reduction, the problem of indirect transition still remains, and the photovoltaic devices cannot be made to have a smaller thickness. In addition, polycrystalline silicon also has problems ascribable to grain boundaries.
Moreover, since it is crystalline, large area wafers cannot be produced, making it difficult to make devices large in area. Hence, in order to withdraw a large electric power, unit devices must be connected by wiring in series or in parallel. Also, expensive packaging is necessary in order to protect photovoltaic devices from mechanical damage caused by various weather conditions when they are used outdoors. This makes production cost higher per unit quantity of electricity generation than existing electricity generation systems. Such problems remain unsettled.
Under such circumstances, for the advancement of bringing photovoltaic devices into practical use for electric power, it is an important technical subject to achieve cost reduction and make devices large in area. Various studies have been conducted, and research has been made on materials such as low-cost materials and materials with high photoelectric conversion efficiency.
Such materials for photovoltaic devices may include tetrahedral type amorphous semiconductors such as amorphous silicon, amorphous silicon germanium and amorphous silicon carbide and compound semiconductors of Group II or VI such as CdS and Cu2S and those of Group III or V such as GaAs and GaAlAs. In particular, thin-film photovoltaic devices in which amorphous semiconductors are used in photovoltaic layers have advantages in that they can provide films having larger area than single-crystal photovoltaic devices, can be formed in a small layer thickness and can be deposited on any desired substrate materials; thus, they are regarded as promising.
However, the photovoltaic devices making use of amorphous semiconductors still have problems with respect to improvement in photoelectric conversion efficiency and improvement in reliability.
As a means for improving the photoelectric conversion efficiency of the photovoltaic devices making use of amorphous semiconductors, for example, the band gap is made narrower so that the sensitivity to long wavelength light can be made higher. More specifically, since amorphous silicon has a band gap of about 1.7 eV, it cannot absorb light having a wavelength of 700 nm or longer and cannot be utilized effectively. Accordingly, employing narrow band gap materials having a sensitivity to long wavelength light is studied.
Such materials may include amorphous silicon germanium, whose band gap can be changed arbitrarily from about 1.3 eV to about 1.7 eV with ease by changing the ratio of silicon material gas to germanium material gas at the time of film formation.
As another method for improving photoelectric conversion efficiency of photovoltaic devices, U.S. Pat. No. 2,949,498 discloses the use of what is called a stacked cell in which photovoltaic devices having unit device structure are superposed in plurality. This stacked cell makes use of p-n junction crystal semiconductors. Its concept is common to both amorphous and crystalline and is suited to make sunlight spectra absorb more efficiently through photovoltaic devices having different band gaps and make Voc (open-circuit voltage) higher so that electricity generation efficiency can be improved.
In the stacked cell, constituent devices having different band gaps are superposed in plurality, and sunlight rays are absorbed efficiently at every part of their spectra so that photoelectric conversion efficiency can be improved. The cell is so designed that what is called the bottom layer, positioned beneath what is called the top layer, has a narrower band gap than the band gap of the top layer, positioned on the light incident side of the constituent devices superposed.
This has enabled sufficient absorption of sunlight spectra to bring about a dramatic improvement in photoelectric conversion efficiency (K. Miyachi et al., Proc. 11th E. C. Photovoltaic Solar Energy Conf., Montrieux, Switzerland, 88, 1992; and K. Nomoto et al., “a-Si Alloy Three Stacked Solar Cells with High Stabilized Efficiency”, 7th Photovoltaic Science and Engineering Conf., Nagoya, 275, 1993).
Now, the above photovoltaic device is a device making use of amorphous semiconductors in all i-type layers, and hence it has had a limit to the prevention of what is called deterioration by light, a phenomenon in which photoelectric conversion efficiency becomes low because of irradiation by light. This is caused by amorphous silicon and amorphous silicon germanium whose film quality has decreased due to deterioration by light and poor carrier movability. This is a phenomenon peculiar to amorphous semiconductors which is not seen in crystal types. Accordingly, under existing circumstances, such a device has a poor reliability and hinders itself from being put into practical use, when used for electric power purposes.
In recent years, research has also been made not only on amorphous/amorphous types but also amorphous/crystalline types, and an improvement in photoelectric conversion efficiency is reported (Hamakawa, Y. et al., “Device Physics and Optimum Design of a-Si/Poly-Si Tandem Solar Cells”, Proceedings of 4th International PVSEC, pp. 403–408, February 1989; (A. Shah, H. Keppner et al., “Intrinsic Microcrystalline Silicon (μc-Si:H)— A Promising New Thin-Film Solar Cell Material”, IEEE First World Conference on Photovoltaic Energy Conversion, pp. 409–412, December 1994; Mitchel, R. L. et al., “The DOE/SERI Polycrystalline Thin-film Subcontract Program, “Processings of 20th IEEE Photovoltaic Specialists Conference, pp. 1469–1476, September 1988).
However, taking account of the balance of electric currents of electricity generated by light in the stacked cell, the cell on the light incident side (the side having a broad band gap) must be formed in a large layer thickness. This has not been satisfactory from the viewpoint of deterioration by light.
Accordingly, it is desired to make amorphous photovoltaic devices undergo much less deterioration by light and to improve their photoelectric conversion efficiency after deterioration by light. In addition, in order to use them for electric power purpose, it is sought to more improve photoelectric conversion efficiency.